Re: [PATCH v2 2/9] KVM: x86: Check CR3 GPA for validity regardless of vCPU mode

[Paolo Bonzini]

On 22/04/21 04:21, Sean Christopherson wrote:
> Check CR3 for an invalid GPA even if the vCPU isn't in long mode.  For
> bigger emulation flows, notably RSM, the vCPU mode may not be accurate
> if CR0/CR4 are loaded after CR3.  For MOV CR3 and similar flows, the
> caller is responsible for truncating the value.
>
> Note, SMRAM.CR3 is read-only, so this is mostly a theoretical bug since
> KVM will not have stored an illegal CR3 into SMRAM during SMI emulation.

Well, the guest could have changed it...

Paolo

> Fixes: 660a5d517aaa ("KVM: x86: save/load state on SMM switch")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>   arch/x86/kvm/x86.c | 11 ++++++++---
>   1 file changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index c9ba6f2d9bcd..63af93211871 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1078,10 +1078,15 @@ int kvm_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3)
>   		return 0;
>   	}
>   
> -	if (is_long_mode(vcpu) && kvm_vcpu_is_illegal_gpa(vcpu, cr3))
> +	/*
> +	 * Do not condition the GPA check on long mode, this helper is used to
> +	 * stuff CR3, e.g. for RSM emulation, and there is no guarantee that
> +	 * the current vCPU mode is accurate.
> +	 */
> +	if (kvm_vcpu_is_illegal_gpa(vcpu, cr3))
>   		return 1;
> -	else if (is_pae_paging(vcpu) &&
> -		 !load_pdptrs(vcpu, vcpu->arch.walk_mmu, cr3))
> +
> +	if (is_pae_paging(vcpu) && !load_pdptrs(vcpu, vcpu->arch.walk_mmu, cr3))
>   		return 1;
>   
>   	kvm_mmu_new_pgd(vcpu, cr3, skip_tlb_flush, skip_tlb_flush);